This invention relates generally to the manufacture of cathode ray tubes (CRTs) and is particularly directed to an improved arrangement and method for incorporating a foil tension mask (FTM) in a color CRT by laser welding.
Recent developments in the area of CRTs have led to the use of shadow masks of the tension foil type in combination with a substantially flat faceplate. CRTs having a flat glass faceplate in combination with an FTM are used in home entertainment television receivers as well as in medium- and high-resolution tubes used as color monitors. Among the various advantages of CRTs using an FTM is an increased power handling capacity permitting as much as a 3-fold increase in display brightness. By maintaining the foil mask, under high mechanical tension in a stretched condition, the FTM does not thermally expand, or dome, under local heating caused by increased electron beam intensities which allow for increased display brightness. By preventing the doming of the FTM, the foil is securely maintained in fixed spaced relation from the faceplate to ensure that the FTM remains in precise alignment within the phosphor dot array on the inner surface of the faceplate.
The stretched FTM is securely mounted to a support frame positioned on or adjacent to the CRT's faceplate. The support frame must be of high strength so as to hold the FTM immovable and maintain it in a highly stretched condition. A tension in the range of 20-40 kpsi is typically applied across the stretched FTM. The FTM is typically affixed to the support frame by welding. U.S. patent application Ser. No. 058,095, cross-referenced above and assigned to the assignee of the present application, discloses means for securing a tension mask to a faceplate-mounted, frame-like shadow mask support structure by laser weldments. U.S. patent applications Ser. Nos. 138,994 and 140,070, also cross-referenced above and assigned to the assignee of the present application, disclose two different means and methods for mapping an FTM receiving surface of a mask support structure for delineating the path of an attachment device such as a laser welding head for permanently affixing an FTM to the support structure. Metallic debris is an unwanted by-product of the welding operation. Even more metallic debris is generated upon severing the FTM from an in-process FTM blank following welding of the FTM to its support structure. The thus produced loose metallic particles tend to land on the FTM, where they can become attached as by lodging in a mask aperture. Obstruction of the mask apertures would, of course, result in video image degradation.
The present invention is intended to eliminate this source of video image degradation by removing the metallic particles produced during FTM welding and cutting. By removing the debris as it is produced during FTM welding and cutting, the metallic particles are prevented from becoming attached to the FTM and blocking its electron beam transiting apertures. By thus maintaining the FTM installation environment essentially free of contaminants, high quality video imagery of the color CRT is ensured.